System failure 1000 when filing return

We are submitting P35 and P14 returns via the Internet using third party software (TPS Webwages/The Payroll Site).

We've had inexplicable authentication errors before but this year we seem to be getting occasional system errors. They don't happen often and submitting the identical return again will generally cure the problem. We've seen these errors both when submitting a return and when listing the documents on the gateway.

Is anyone else getting these errors and is there a trend we can spot? I suspect that the errors happen on days when their systems are particularly busy, which makes me nervous about how it is going to cope next month.

The error returned by the government gateway is
Error number 1000 "System failure. The submission of this document has failed due to an internal system error."

I've spoken to HMRC - both the technical services helpdesk and the SDS team and they are aware some people are getting this error but they don't know what is causing it. They suggest submitting the return again until it suceeds, which seems to work so far.
Steven Tucker